Hydrodynamic constraints on the energy efficiency of droplet electricity generators

Electric energy generation from falling droplets has seen a hundred-fold rise in efficiency over the past few years. However, even these newest devices can only extract a small portion of the droplet energy. In this paper, we theoretically investigate the contributions of hydrodynamic and electric losses in limiting the efficiency of droplet electricity generators (DEG). We restrict our analysis to cases where the droplet contacts the electrode at maximum spread, which was observed to maximize the DEG efficiency. Herein, the electro-mechanical energy conversion occurs during the recoil that immediately follows droplet impact. We then identify three limits on existing droplet electric generators: (i) the impingement velocity is limited in order to maintain the droplet integrity; (ii) much of droplet mechanical energy is squandered in overcoming viscous shear force with the substrate; (iii) insufficient electrical charge of the substrate. Of all these effects, we found that up to 83% of the total energy available was lost by viscous dissipation during spreading. Minimizing this loss by using cascaded DEG devices to reduce the droplet kinetic energy may increase future devices efficiency beyond 10%.

A Well Flux Surveillance and Production Ramp-Up Method for Openhole Standalone Screen Completion

Summary Well surveillance requires practical models to balance the reward of maximizing production with the risk of ramping up production too much, which damages the completion. In this paper we present a method to monitor and ramp up production for openhole standalone screen (OH-SAS) completion. The objective is to optimize production using pressure transient analyses to assess the completion impairment and failure risks during the production ramp-up process. The flux model incorporates filter-cake pinholes, which are formed from nonuniform deposition and cleanup of filter cake during drilling and completion operations. Pinholes cause concentrated fluxes and increase completion failure risks. The method comprises three components, which are (1) determine pinhole properties from laboratory tests, (2) relate completion pressure drop of production through pinholes to pressure transient analyses, and (3) distribute fluxes in the standalone screen wellbore. Examples are presented and show that the completion pressure drop as a function of flow rate is nonlinear and higher with pinholes than without pinholes. By not incorporating pinholes, operations can potentially limit ramp-up. Flux distribution examples show that the largest impingement or radial velocity is at the top section of screen. The axial annular flow velocity or scouring velocity is two orders of magnitude larger than the screen impingement velocity. An integrated flux surveillance method for OH-SAS completion is presented for field applications.

Experimental and Numerical Investigations on Jet Impingement Cooling for Electronic Modules

In this paper, the influence of outlet arrangement and plenum structure on impingement cooling is experimentally and numerically investigated in a typical 1-U confined server space. Three outlets include Z-type, bilateral, and U-type arrangements, and the plenum configurations contain partially inclined, fully inclined, and staged layouts. As a result, using the U-type outlet or staged plenum may prominently compromise the impingement cooling performance on the target plates with lower pumping power. With numerical investigation, it is found that, for the case with Z-type outlet, the flowrate of jet impingement increases alongside the streamwise direction. Besides, the impingement stagnation region on target plates with the minimum thermal resistance may shift toward the outlet. Meanwhile, the uniformity of jet impingement can be improved by 10.7% and 50.3% when the bilateral and U-type outlets are applied, respectively, and the jet impingement is changed to perpendicular direction due to the opposite cross flow from the coming flow direction. On the other hand, by applying the inclined plenum and staged plenum, the uniformity of jet impingement can be dramatically improved by 113.9% and 215.1%, respectively. However, the local jet impingement velocity distribution is still nonuniform. Hence, a novel design of impingement plate based on the concept of Coanda effect is proposed. The peak value of the thermal resistance on target plate can be reduced by 21.8% and 16.0% at the center region and the fore part of the jet array.

The Effect of Water Droplet Size, Temperature, and Impingement Velocity on Gold Wettability at the Nanoscale

Molecular dynamics (MD) simulations are performed to investigate the wettability of gold substrate interacting with nanosized droplets of water. The effects of droplet size, temperature variation, and impingement velocity are evaluated using molecular trajectories, dynamic contact angle, spread ratios, radial distribution function (RDF), and molecular diffusion graphs. Droplets of 4 nm and 10 nm were simulated at 293 K and 373 K, respectively. Stationary droplets were compared to droplets impinging the substrate at 100 m/s. The simulations were executed on high-end workstations equipped with NVIDIA® Tesla graphical processing units (GPUs). Results show that smaller droplets have a faster stabilization time and lower contact angles than larger droplets. With an increase in temperature, stabilization time gets faster, and the molecular diffusion from the water droplet increases. Higher temperatures also increase the wettability of the gold substrate, wherein droplets present a lower contact angle and a higher spread ratio. Droplets that impact the substrate at a higher impingement velocity converge to the same contact angle as stationary droplets. At higher temperatures, the impingement velocities accelerate the diffusion of water molecules into vapor. It was revealed that impingement velocities do not influence stabilization times. This research establishes relationships among different process parameters to control the wettability of water on gold substrates which can be explored to study several nanomanufacturing processes.

Rotary Disk Atomization

In the late 1930s, European engineers discovered that, for very low flow rates, rotary disk atomizers produced a more definable range of droplet sizes than hydraulic atomizers. In the late 1970s, a cup-like spinning atomizer was developed to apply herbicides at low and ultra-low volumes. Rotary atomizers distribute droplets in a pattern similar to hollow cone nozzles. The droplet trajectory could affect deposits adversely since droplets released horizontally are exposed to wind and other environmental effects longer than hydraulic spray nozzles. Propellers and fans were used to enhance downward movement of droplets without considering that droplet impingement velocity was critical for efficient deposition. In the early 1980s, rotary atomizers were promoted to reduce herbicide rates, but the claims were products of unconfirmed testing. Herbicide efficacy in confirmed research was not influenced by application with the rotary atomizer, but lower carrier rates reduced the amount of water handled during the spraying operation. The cost of the atomizer, more maintenance, and greater care during operation with no decrease in herbicide rates discourage continued use.

Measured Particle Rebound Characteristics Useful for Erosion Prediction

Extensive high temperature particle rebound testing has been continuing at the University of Cincinnati for the past three years utilizing the new high temperature material erosion facility. Particle rebound characteristics including particle angle and velocity restitution ratios are important in the prediction of particle trajectories through a turbomachine or other flow devices where particulate erosion is present and the prediction of erosion patterns is desired. This paper presents particle rebound data which has been accumulated for 160 microns particles impinging on INCO 718, Ti 6-4 and 2024 AL targets. Particle rebound characteristics are compared with important parameters such as target temperature, particle impingement velocity and particle impingement angle. In addition a semi-empirical erosion rate equation for INCO 718 material is derived.